1. Field of the Invention
The present invention relates to an optical transmission assembly which reduces electromagnetic noise between transmission and reception.
2. Description of the Related Art
Optical signals may be transmitted/received in a longitudinal direction of a core by surface-mounting a light emitting device or a light receiving device on an optical circuit substrate with the core formed between claddings, forming reflective surface partially in the core, and optically coupling the light emitting or receiving device and the optical axis of the core.
A conventional optical transmission assembly of this kind is designed to combine a transmitted light signal and a received light signal, and transmit to a common core extending to an input/output port. For example, optical transmission assembly 51 shown in FIGS. 5 and 6 comprises a light emitting device 53 and a light receiving device 54 mounted on the upper surface of a cladding 52, and a core 55 formed in the cladding 52 that is passed below each of the light emitting device 53 and the light receiving device 54. A reflective surface 56 is formed by forming, e.g., dicing an inverted V-shape groove that extends from the lower surface of the cladding 52 to the core 55, the inclined surface of the groove via which the core 55 and a material (herein, air) with a refractive index smaller than that of the core 55, being in contact with each other.
The light emitting device 53 side core 55 and the light receiving device 54 side core 55 intersect at a specified angle in top view. At that crossing point, a square groove 57 extending from the upper surface of the cladding 52 to the core 55 is diced by being intersected at a specified angle to the core 55 in top view. A wavelength filter 58 is inserted in the square groove 57. The transmitted light signal and received light signal may be combined (separated) according to angles made by each core 55 and wavelength filter 58. An optical transmission assembly on the other side not shown has the same structure, except that only the mode of wavelength filter 58 is different in both the optical transmission assemblies.
For example, in the optical transmission assembly 51 on one side shown, to transmit wavelength 1.5 μm light and receive wavelength 1.3 μm light, the wavelength filter 58 is installed that reflects the wavelength 1.3 μm light and transmit wavelength 1.5 μm light, while in the optical transmission assembly on the other side not shown, to transmit wavelength 1.3 μm light and receive wavelength 1.5 μm light, the wavelength filter 58 is installed that reflects the wavelength 1.5 μm light and transmit wavelength 1.3 μm light.
In the optical transmission assemblies on both sides, this allows the light signal transmitted from one optical transmission assembly to be transmitted through its own wavelength filter 58 to the other optical transmission assembly, while allowing the light signal transmitted from the other optical transmission assembly to be reflected and divided at its own wavelength filter, and received. In this manner, bidirectional transmission/reception is possible at the common input/output port (See, JP-A-2005-91460, JP-A-2003-139979).
In the optical transmission assembly 51 of FIG. 5, the light emitting device 53 and the light receiving device 54 are both mounted on the upper surface of the cladding 52. The driver 60, which drives the light emitting device 53, and the preamplifier 59, which amplifies received light signals of the light receiving device 54, are also mounted on the upper surface of the cladding.
The driving current for the light emitting device 53 is on the order of a few tens of mA, while the received light signals of the light receiving device 54 are on the order of a few μA, which is small by the order of 3 digits compared to the driving current. For this reason, electromagnetic noise from the driver 60 adversely affects operation of the preamplifier 59. Namely, amplifying a faint received signal with a few hundred-fold to a few tens-of-thousand fold gain in the preamplifier 59 likewise causes amplification of electromagnetic noise components, resulting in deterioration in signal-to-noise ratio.
The simplest method to avoid electromagnetic noise is to make large the distance between the driver and the preamplifier. However, in order for the optical transmission assembly to be small-sized and integrated, it is desirable to avoid making large the distance between the light emitting device and the light receiving device.